1. Field of the Invention
The present invention relates to illumination optical systems.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can comprise a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
Illumination modes and beam shaping (e.g., field sizes and shapes) are typically generated by utilization of diffractive and/or refractive arrays that form diffractive or refractive beams of radiation. Desired modification of the diffracted or refracted beams produced by the respective diffractive or refractive arrays is achieved using full field non-imaging (e.g., condenser) and imaging (e.g., relay) optical systems.
For lenses or the arrays (e.g., lens arrays), the resolving power depends upon a wavelength of light being used and inversely upon a numerical aperture. The numerical aperture is the product of a refractive index, n, of a medium (e.g., n=1 for air, n=1.5 for immersion fluid, etc.) and a sine of an angle, i, a semi angle of a cone formed by joining objects to a perimeter of the array. The larger the value of numerical aperture, the better the resolving power of the lens or array.
In case of optical systems with high magnification (or demagnification), such as maskless lithography and microscopy, illumination output beams with very low divergence (numerical aperture) are required. This means that: (a) the input beam divergence (pre-numerical aperture) has to be very low (e.g., at least 3-4 times smaller compared with that of the output beam), which is sometimes unachievable, and (b) the feature sizes of the diffractive or refractive arrays have to be large.
In case of high numerical aperture optical systems, such as in immersion lithography tools, illumination output beams with high divergence (numerical aperture) are required. Two main problems in this case are: (a) the feature sizes of the diffractive or refractive arrays have to be very small, which poses manufacturing difficulties when manufacturing these arrays; and (b) the inability of a condenser in the optical system to fulfill both field size and beam divergence requirements simultaneously.
What is needed is an optical system and method, which may be used in an illumination system of a lithography system, that produces desired numerical apertures or divergences of illumination radiation produced by refractive or diffractive arrays.